Immersion process for heavy underwater structures with particular applications for underwater oil tanks

ABSTRACT

This invention relates to an immersion process for structures whereby the structure to be immersed is equipped with at least two groups of floats each comprising at least two individual floats in which, by using elongated floats attached vertically to the structure to be immersed and capable of being ballasted or unballasted by the filling or emptying of a central compartment forming the principal ballast, an operation is carried out alternately on each one of the floats in each group consisting either in the ballasting of the said float followed by unballasting and by the simultaneous release of a given length of the chain or cable connecting the structure to the float, or solely in the release of a given length of the chain or cable, the associated float being empty and its position in relation to the structure being fixed during this operation, and in which this operation is carried out simultaneously or alternately on each group of floats until the structure is immersed in the proper position. Both the safety and the performance of the immersion system are thus improved.

United States Patent 91 Lafiont [451 Sept. 4, 1973 [75] Inventor: Maurice Laftont, La Celle-St.-Coud,

France [73] Assignee: Entreprise de Recherches et dActivites Petrolieres (ELF), Paris, France [22] Filed: July 23, 1971 [21] Appl. No.: 165,526

[30] Foreign Application Priority Data July 24, 1970 France 7027350 [52] US. Cl 11 4/05 T, 9/8 R, 114/05 F [51] Int. Cl B63b 35/02, 865d 89/10 [58] Field of Search 114/0.5 T, 0.5 R, 114/0.5 F, 0.5 D, 16 E, 16.8, 206 R; 9/8 R; 61/465, 72.1

[56] References Cited UNITED STATES PATENTS 1,840,324 1/1932 Lindquist 114/05 R 2,118,466 5/1938 Jennings 114/168 2,371,404 3/1945 Mumford l14/0.5 T 2,783,027 2/1957 Gilbert 9/8 R X 2,908,141 10/1959 Marsh 114/206 R X 3,189,922 6/1965 Margot et a1. 9/8 R 3,256,537 6/1966 Clark 9/8 R 3,386,407 6/1968 Mount 1 14/206 R 3,559,413 2/1971 Silverman 114/05 R 3,623,443 11/1971 Luther 114/16 E Primary Examiner-Milton Buchler Assistant Examiner-Barry L. Kelmachter Att0rney-Holc0mbe, Wetherill & Brisebois [5 7] ABSTRACT This invention relates to an immersion process for structures whereby the structure to be immersed is equipped with at least two groups: of floats each comprising at least two individual floats in which, by using elongated floats attached vertically to the structure to be immersed and capable of being ballasted or unballasted by the filling or emptying of a central compartment forming the principal ballast, an operation is carried out alternately on each one of the floats in each group consisting either in the ballasting of the said float followed by unballasting and by the simultaneous release of a given length of the chain, or cable connecting the structure to the float, or solely in the release of a given length of the chain or cable, the associated float being empty and its position in relation to the structure being fixed during this operation, and in which this operation is carried out simultaneously or alternately on each group of floats until the structure is immersed in the proper position. Both the safety and the performance of the immersion system are thus improved.

8 Claims, 6 Drawing Figures PAT'ENTEDSEP 4 Ian SHEET 1 OF 3 PATENTEDSEP 4 1975 SHEET 3 OF 3 IMMERSION PROCESS FOR HEAVY UNDERWATER STRUCTURES WITH PARTICULAR APPLICATIONS FOR UNDERWATER OIL TANKS The object of this invention is a process and devices for the immersion of heavy structures; it is applied in particular, but not exclusively, to underwater tanks for the storage of hydrocarbons.

The problem to be resolved is, after transporting such a structure to the site at which it is to be immersed, how to lower it slowly beneath the surface of the water while maintaining it continuously stable. In addition, safety must be absolute. Existing processes, such as those defined in French Pat. No. 1,547,703, make use of chains of floats hooked to the structure, these floats being successively filled with water to enable the entire structure to sink beneath the water.

In order for this system to be stable, it is necessary to have several series of floats in a star-shaped arrangement around the structure, and preferably two groups of series of floats, so that one of the series is in positive flotation while the floats of the other group are filling.

In any case, this system requires an extremely large number of floats, and hence involves a substantial investment. In addition, its implementation requires an unencumbered space whose radius is at least equal to the depth at the immersion site.

Finally, in the event of a mistake or in an emergency, it is extremely difficult to stop the immersion process and perform the inverse operations.

The structure immersion process according to the present invention, whereby the structure to be immersed is equipped with at least two groups of floats, each of which includes at least two floats, is characterized in that each of the floats, which are elongated in shape, are attached vertically to the structure to be immersed by a chain or cable of variable length, and are able to be ballasted or unballasted as required via the filling or emptying of the central compartment forming the main ballast. An operation is performed alternately on each of the floats in each group consisting either of the ballasting of the said float followed by unballasting and the simultaneous releasing of a given length of the chain or cable connecting the structure to its float, or solely'in the releasing of a given length of the chain or cable connecting the structure to this float, the associated float remaining empty and its position in relation to the structure remaining fixed throughout this operation. This operation is carried out simultaneously or alternately on each group of floats until the structure is immersed in the proper position.

The structure immersion device for the implementation of the process according to the invention includes at least two groups of floats each of which consists of at least two floats attached to the structure to be immersed, each of the floats, which are of elongated shape, including a central compartment forming the main ballast, a lower compartment the filling of which ensures the vertical positioning of the float, a chainwell in the lower compartment, an upper compartment connected with the chain-well via a conduit that traverses the central compartment, and connected with the outside via a central chimney traversing the central compartment and the lower compartment, said upper compartment supporting a windlass over which passes a chain held in reserve in the chain-well and which can be attached to the structure passing through the central chimney; means for filling and emptying the central compartment and the lower compartment, and means for varying the chain length.

In one form of this device, the means for filling and emptying the central compartment and the lower compartment include a compressed air inlet connected by a gate system to each of the said compartments, and two conduits, each of which admits water to one of the compartments through gates controlled from the upper level.

The invention will be better understood through the description that follows, illustrated by the accompanying figures.

FIG. 1 represents the cross section of a float.

FIG. 2 shows the manner in which the floats are attached to the tank.

FIG. 3 represents the starting phase of the immersion of the tank.

FIGS. 4 and 5 represent two complementary intermediate phases of the immersion.

FIG. 6 shows the end of the immersion.

FIG. 1 shows a float designed to equip the structure for its immersion. This float is of elongated shape and includes three compartments. Compartment 1 has a large volume and extends along about three quarters of the length of the float; compartment 2, in the lower part, is independent from compartment 1. Compartment 2 has a chain-well 4 in which a certain length of chain is held in reserve. The upper part of the float houses compartment 3, which has a sufficient volume so that, when compartments 1 and 2 are full of water, it ensures the buoyancy of the total structure. In the upper part of compartment 3, there is a hydraulic braking pulley 6, or windlass, over which passes chain 5 coming from chain well 4.

A brake for chain 7 is also mounted in the upper part of compartment 3. The chain traverses this brake and descends via a central chimney that runs through the entire float, this chain being attached in a working position to the structure to be immersed.

A compressed air inlet 9 makes it possible, via pipe 10, to empty compartment 2 and, via pipe 11, to empty main compartment 1. Two gates 13 and 13 make it possible to let water into or evacuate it from compartments 1 and 2. The gates are operated from the upper level by controls 14 and 15.

FIG. 2 represents the hooking of a float onto the structure. This structure is brought to the immersion site in a state of positive buoyancy, towed by towers. At the immersion site, structure 16 is equipped with a sufficient number of auxiliary floats 17. When the auxiliary floats are attached, the structure is given a slightly negative buoyancy so that the apparent weight is supported by auxiliary floats 17. At this juncture, the chains of the various floats are hooked onto the anchorage points 25. Compartment 2 is then ballasted, thereby enabling the float to assume a vertical position and, using the windlass described above, considerable tension is applied to chain 23. Two floats represented by 19 and 22 are shown clearly emerging above the surface of the water 18.

In addition, the structure to be immersed is equipped with cables 20 and 21 attached to chambers not shown, thereby ensuring considerable lateral tension designed to center the structure on the immersion site and prevent it from sliding sideways.

FIG. 3 shows an intermediary phase in the immersion.

Floats 17 have been removed and the two floats 19 and 22- are in a vertical position. Main compartment 1 of float l9 and compartment 2 of float 22 have been filled with water. In this way, float 19 has been forced down, enabling the structure to sink to a certain depth. Float 22 is in a position right on the surface of the water and is supporting the structure. This cross section shows only two floats, but in actual practice, two, three or four series of two floats are distributed equiangularly around the structure. The floats function in pairs, as shown in the figures.

The brake is then released, enabling the chain to be let out to a given length, with the result that there is a lessening of the tension exerted on structure 16 by chain 23.

, Simultaneously, at a reasonable rate of speed, compartment 1 of float 19 is emptied. Under the effect of these two operations, float 19 rises and float 22, whose main compartment is empty, sinks.

When the point of equilibrium has been reached, water is let in to fill up the main compartment of float 22, which in turn sinks, enabling the structure to descend, while float 19, which also sinks and exerts an increasing tension on the structure, thus braking its descent. The safety position is therefore continuously maintained, with at least one series. of floats held in readiness to brake the structure in the event the latter should sink as the result of a wrong move.

FIG. 4 shows the phase during which float l9, whos main compartment is filled with water, has completed its run, while float 22, whose main compartment is empty, is braking the structures downward motion. At this point, chains 24 and 23 are under tension. The ensuring operation consists of releasing a given length of chain 23 while emptying, by the use of compressed air, the main compartment of float 19. Float 19 rises, while float 22 sinks.

When the main compartment of float 19 has become empty, chain 19 is again braked and water is let in to fill the main compartment of float 22, which therefore sinks slowly, simultaneously causing the sinking of float 19, which is empty. This operation is shown in FIG. 5 in the phase during which the floats are empty.

FIG. 6 shows the following phase during which the main compartment of float 22 is full while the main compartment of float 19 is empty. We thus gradually get nearer to the immersion of the structure, using a step-by-step approach, and continuously exerting a considerable safety traction so as to prevent the maneuver from geting out of hand at any time. This safety is automatic.

Traction is simultaneously exerted on cables 20 and I 21, which are attached to the centering chambers,

these chambers being drawn near to the center of the immersion circle, cables 20 and 21 having a constant length if the spacing of the anchorage chambers has been correctly regulated.

By alternately emptying and filling compartments 1 of floats l9 and 22, it is possible to improve both the safety and the performance of the immersion system.

What is claimed is:

l. A method of immersing a structure in a body of water which comprises the steps of utilizing flexible connecting means to attach to said structure a plurality of pairs of elongated floats,

each float comprising at least one ballast compartment and being equipped with means for varying the effective length of said connecting means between said structure and float and means for ballasting and unballasting said compartment,

introducing a larger quantity of ballast into the ballast compartment of one of said floats than into the other,

removing said larger quantity of ballast from said one float while increasing the effective length of the connecting member connecting said one float to said structure, and

repeating said ballast introducing and removing steps until said structure reaches the desired position.

2. A method as claimed in claim 1 which comprises the step of simultaneously introducing and removing ballast from one float of each pair.

3. A method as claimed in claim 1 which comprises the step of first introducing and removing ballast from one float of one pair, and next introducing and removing ballast from one float of another pair.

4. The method claimed in claim 1 which comprises the step of simultaneously supporting said structure by means of lines connected to additional floats initially positioned at points substantially more remote from said structure then said pairs of floats, which additional floats are drawn toward a position above said structure as said structure descends.

5. A structure immersion assembly consisting of at least two groups of floats each of which has at least two floats attached to the structure to be immersed, characterized by the fact that each of the floats, which are of elongated shape, includes a central compartment forming the main ballast tank, a lower compartment the filling of which ensures the vertical positioning of the float, a chain-well in the lower compartment, an upper compartment connected with the chain-well via a passage traversing the main compartment and connected to the outside via a chimney traversing the main compartment and the lower compartment, the said upper compartment containing windlass over which there passes a chain held in reserve in the chain-well and capable of being attached to the structure by passing through the central chimney, means for filling and emptying the central compartment and the lower compartment, and means for regulating the length of the chain.

6. A structure immersion assembly according to claim 5 characterized by the fact that the means for filling and emptying the central compartment and the lower compartment of each float include a compressed air inlet connected by a gate system with each of the said central and lower compartments and two watercarrying conduits, each of which conduits is connected with one of the compartments viaremotely controlled gates.

7. An assembly in accordance with claim 5 characterized by the fact that the means for regulating the chain length comprises a brake in the upper compartment at the entrance of the chimney.

8. An assembly according to claim 5 characterized by the fact that the volume of the upper compartment is sufficient to ensure the buoyancy of the float when both the central compartment and the lower compartment are full. 

1. A method of immersing a structure in a body of water which comprises the steps of utilizing flexible connecting means to attach to said structure a plurality of pairs of elongated floats, each float comprising at least one ballast compartment and being equipped with means for varying the effective length of said connecting means between said structure and float and means for ballasting and unballasting said compartment, introducing a larger quantity of ballast into the ballast compartment of one of said floats than into the other, removing said larger quantity of ballast from said one float while increasing the effective length of the connecting member connecting said one float to said structure, and repeating said ballast introducing and removing steps until said structure reaches the desired position.
 2. A method as claimed in claim 1 which comprises the step of simultaneously introducing and removing ballast from one float of each pair.
 3. A method as claimed in claim 1 which comprises the step of first introducing and removing ballast from one float of one pair, and next introducing and removing ballast from one float of another pair.
 4. The method claimed in claim 1 which comprises the step of simultaneously supporting said structure by means of lines connected to additional floats initially positioned at points substantially more remote from said structure then said pairs of floats, which additional floats are drawn toward a position above said structure as said structure descends.
 5. A structure immersion assembly consisting of at least two groups of floats each of which has at least two floats attached to the structure to be immersed, characterized by the fact that each of the floats, which are of elongated shape, includes a central compartment forming the main ballast tank, a lower compartment the filling of which ensures the vertical positioning of the float, a chain-well in the lower compartment, an upper compartment connected with the chain-well via a passage traversing the main compartment and connected to the outside via a chimney traversing the main compartment and the lower compartment, the said upper compartment containing windlass over which there passes a chain held in reserve in the chain-well and capable of being attached to the structure by passing through the central chimney, means for filling and emptying the central compartment and the lower compartment, and means for regulating the length of the chain.
 6. A structure immersion assembly according to claim 5 characterized by the fact that the means for filling and emptying the central compartment and the lower compartment of each float include a compressed air inlet connected by a gate system with each of the said central and lower compartments and two water-carrying conduits, each of which conduits is connected with one of the compartments via remotely controlled gates.
 7. An assembly in accordance with claim 5 characterized by the fact that the means for regulating the chain length comprises a brake in the upper compartment at the entrance of the chimney.
 8. An assembly according to claim 5 characterized by the fact that the volume of the upper compartment is sufficient to ensure the buoyancy of the float when both the central compartment and the lower compartment are full. 